Hydraulic systems usually convert fluid pressure to linear force by applying the fluid pressure to an end of a cylindrical piston which slides axially in a matching bore. A piston rod extends from the piston or shaft out through at least one end of the bore. To avoid loss of fluid and fluid pressure from the system, it is necessary to provide a seal between the piston rod and the end of the bore.
Typical fluid seals for use in hydraulic systems include a metal "cup" having a circular opening across most of its bottom. An elastomeric material is bonded around the inside of the metal cup and extends radially inwardly to define a lip encircling the opening in the cup. In use, the cup is pressed into a suitable recess at the end of the cylindrical bore and the lip extends into the bore to press against the sides of the piston rod and take up any space between the bore and the piston rod. To ensure that the lip presses against the piston rod and that the lip maintains its shape, the lip has a groove extending around it behind its sealing face and an annular metal spring is mounted in the groove.
Although attempts have been made to eliminate the metal cup and make the seal entirely of a resilient material, such attempts have previously met with little success. The sealing face of a seal is preferably relatively soft to contour around the cylindrical piston rod and minimize leakage. In some applications, such a soft seal without a metal cup will not be retained strongly enough in the cylindrical bore and will be dislodged as the piston rod is moved or if suction is applied. An example of an application where soft resilient seals have been found ineffective is in automobile power steering racks. Although a harder seal will not be dislodged or sucked out, harder seals do not usually have adequate sealing capabilities.
In addition, in power steering racks the piston rod is sometimes subjected to radial forces. When this occurs, the rod tends to distort the seal and the shape of the sealing lip. Such distortion can cause the lip to lose sealing engagement with the rod, resulting in the loss of fluid and fluid pressure from the power steering system.
It is an object of the present invention in one aspect to provide a fluid seal having a plurality of surfaces for engaging the movable member in a fluidly sealing manner and to guide the movable member through the seal.
It is a further object of the present invention to provide a fluid seal without a metal cup which will nevertheless resist being dislodged by relative axial movement between the components being sealed.
It is a still further object of the present invention to provide a fluid seal which is resistant to abrasive wear by dirt adhering to the surface of the moveable member.